1. Field of the Invention
The present invention relates to a frequency discriminator that cooperates with an arrangement for measuring the frequency of an applied RF signal. More particularly, the present invention relates to a frequency discriminator cooperating with an arrangement that substantially eliminates frequency measurement delay errors typically caused by temperature variations in a delay line used to measure the frequency of the applied signal. Specifically, the present invention relates to an arrangement that measures the resistance of the delay line carrying the RF signal being measured and uses the measured resistance to compensate for the temperature variations of the delay line that would otherwise degrade the accuracy of the frequency measurement.
2. Description of the Prior Art
RF radio receivers that measures the frequency of an applied RF signal commonly employ a frequency discriminator. These RF radio receivers are usually characterized as digital frequency discriminators (DFD), or instantaneous frequency measurement (IFM) receivers, both known in the art. These RF radio receivers employed for RF frequency measurements commonly use one or more correlators also known in the art. The RF radio receivers that utilize correlators serve well their intended purpose, but do suffer accuracy degradation due to temperature variations and such radio receivers may be further described with reference to FIGS. 1-6 illustrated herein.
FIG. 1 illustrates a prior art arrangement 10 of a RF radio receiver comprising a frequency discriminator 12 that receives a RF signal 14 having a radian frequency, .omega., and delivers an output signal 16 representative of a vernier or coarse measurement of the frequency characteristic of the applied RF signal 14.
The RF signal 14 is also applied to a correlator 18. The correlator 18 directs the RF signal into two paths, the first path being a reference path and the second path being a delay path provided by a delay device 20 whose characteristic provides for a path length difference between the delay and reference paths indicated by the parameter .tau. and measured in seconds. As is known in the art, the correlator 18 operates in a manner analogous to performing mathematical manipulations so as to produce one or more product signals comprising the RF signal delayed by the path provided by the delay device 20 times the RF signal that is directed down the reference path and does not encounter the delay device 20. These products are illustrated in FIG. 1 as signals 22 and 24, each of which contain the quantities .omega. and .tau.. The signals 22 and 24 respectively representative of the phase shift encountered by the RF signal 14 directed down the reference path, and the phase shift encountered by the RF signal 14 directed down the delay path, more particularly, encountered by the delay device 20. The product signals 22 and 24 are provided as output voltages from the correlator 18 and respectively are representative of and proportional to the sine and cosine of the phase angle difference of the RF signal 14 as measured between the delay and reference paths of the correlator 18. The correlator 18 provides at least one of the product signals 22 and 24, but it is preferred that both product signals 22 and 24 be provided. The product signals 22 and 24 may be respectively represented as sin .omega..tau. and cos .omega..tau., as shown in FIG. 1.
The delay line 20 may be implemented by a coaxial cable, printed delay line or some other delay device, and by choosing the proper characteristic of the delay line path (as known in the art) a desired interrelationship between the RF input frequency of the RF signal 14 to a video output (to be described with reference to FIG. 9) can be achieved. Moreover, improved measurement resolutions and accuracy of the RF frequency may be provided (as known in the art) by the use of multiple correlators 18, to be further described hereinafter with reference to FIG. 5, arranged to perform mathematical manipulations which correspondingly provide for more accurate product output signals 22 and 24. However, in spite of further accuracy improvements for the product signals 22 and 24, the use of the delay line 20 degrades the accuracy of the RF frequency measurement.
The accuracy of the RF frequency measurement signal is a direct function of the accuracy of the RF delay line 20. The delay line 20 typically exhibits a repeatable error characteristic which is temperature dependent. To eliminate this temperature dependent error, a variety of approaches have been employed to compensate for changes in temperature and its attendant degradation of the accuracy of the frequency measurement and one such approach may be described with reference to FIG. 2.
FIG. 2 illustrates a prior art arrangement 26 which is quite similar to that of FIG. 1 with the exception that an oven 28 has been added thereto and in which is arranged the delay device 20. The delay device 20 is not subjected to any accuracy degrading temperature variations, because of the constant temperature provided by the oven 28. The arrangement 26 having the oven 28 serves well its intended purpose, but has the attended drawbacks of power consumption, the need of warm-up time to allow the temperature of the oven to stabilize before the operation of circuit arrangement 26 is initiated, and also the disadvantage of the unreliability of the oven 28. Furthermore, as previously mentioned, when employing multiple correlators, such as those of FIG. 5, to be further described hereinafter, placed into a parallel array and arranged in a fixed ratio, relative to each other and having typical values of 2:1; 3:1 or 4:1, all of the temperatures of all of the RF delay lines are usually not stabilized. When such is the case, the characteristics of the RF delay lines will differ between the oven stabilized lines and those that are not. This difference is accentuated over temperature, and can lead to temperature tracking errors and large errors in the output frequency data measurements. An arrangement that does not suffer the inherent oven drawbacks may be described with reference to FIG. 3.
FIG. 3 illustrates a prior art arrangement 30 which is similar to that of FIG. 1 except that the conventional delay line 20 of FIG. 1 is replaced with a relatively large (compared to delay line 20) coaxial delay line 32, known in the art, having a low temperature coefficient. The arrangement 30 substantially corrects the temperature variation degradation of the accuracy of measuring frequency without suffering the disadvantages of the oven 28 of FIG. 2, but it does suffer the disadvantages of a higher cost (relative to delay line 20 ) and also packaging problems because of its relatively large size. Furthermore, because the special low temperature coefficient coaxial cable delay lines 32 are, in general, factory assembled, it causes field maintenance problems. An arrangement that does not suffer the disadvantages of either FIGS. 1, 2 or 3 may be further described with reference to FIG. 4.
FIG. 4 illustrates a prior art arrangement 34 which has many of the features of FIGS. 1-3 including a frequency discriminator 12 that receives the RF input signal 14 and provides an analog output signal representative of the frequency of the RF signal which, for arrangement 34, is routed to an analog to digital converter 40. The analog to digital converter 40 provides a first digital signal 16A representative of data that corresponds to the coarse frequency of the RF signal 14 which is routed to a programmable memory device 42. The programmable memory device 42, to be further described hereinafter with reference to FIG. 6, may be a conventional processor that is responsive to a program that causes the programmable memory device 42 to operate in a predetermined manner to combine digital signals. FIG. 4 further illustrates the arrangement 34 as having an analog to digital converter 44 which receives the product signals 22 and 24 and converts them into respective digital data which are routed to the programmable memory device 42. The .tau. quantity of the product signals 22 and 24 sine .omega..tau. and cosine .omega..tau., respectively, is produced by a delay line 46.
The delay line 46 may be either a coaxial cable, a printed delay line, or another equivalent device, all known in the art, which cooperates with a temperature sensitive resistor 48 that is thermally coupled to the delay line 46 by means of a thermally conductive compound 50, all known in the art. The temperature sensitive resistor 48 has its first end connected to a first end of resistor RT1 which has its second end connected to ground and, further, the temperature sensitive resistor 48 has its second end connected to a first end of resistor RT2 which has its second end connected to a voltage reference 53. The temperature sensitive resistor 48 develops a voltage thereacross that is applied to an operational amplifier 52, by means of signal paths 54 and 56. The operational amplifier 52 amplifies the received signal and develops an output voltage, sometimes referred to as a video temperature voltage, which is applied to an analog to digital converter 52A. The analog to digital converter 52A provides a second digital signal representative of the temperature being sensed by the delay line 46.
The programmable memory device 42 serves as a combiner to combine the data generated by the analog to digital converter 52A, serving as temperature dependent additive factor, with the data contained in signal 16A (coarse frequency data) so as to provide frequency and temperature dependent data to produce an accurate measurement signal 60 of the frequency of the RF signal 14 that is compensated for temperature errors. The arrangement 34 of FIG. 4 may be further described with reference to FIGS. 5 and 6 associated with digital frequency discriminators (DFD) and with FIG. 5 illustrating a general arrangement 36A and FIG. 6 illustrating an arrangement 38B showing some of the essential details of FIG. 5.
FIG. 5 illustrates the arrangement 36A as comprising seven correlators, indicated as 18A, 18B, . . . 18F and 18G, with correlator 18G having the parallel temperature sensing resistor 48 indirectly coupled to the longest RF device line, that is, the delay line of the correlator 18G. The correlators 18A, 18B, 18F and 18G receive their RF signal 14 from a power divider 38 that receives the incoming RF signal 14. Each of the correlators 18A, 18B, . . . 18F and 18G produces the product signals 22 and 24 (sin .omega..tau. and cos .omega..tau.) which are routed to the arrangement 36B of FIG. 6.
FIG. 6 illustrates the programmable memory device 42 as comprised of elements 42A (TTL comparators and latches), 42B (error correction prom), 42C (temperature correction prom), 42D (output data latch), and 42E (phase split prom). FIG. 6 further illustrates amplifier 44A and 44B that respectively receive the sin .omega..tau. and cos .omega..tau. quantities of the seventh correlator 18G.
The differential sine (sin .omega..tau.) and cosine (cos .omega..tau.) video outputs from correlators 18A through 18F are provided to TTL comparators and latches 42A. This provides a 12-bit address input to the error correction prom 42B. In parallel, the sine (sin .omega..tau.) and cosine (cos .omega..tau.) video from the seventh (and longest delay) correlator 18G is amplified by video amplifiers 44A and 44B and are provided to ADC (Analog to Digital Converter) 44. The Most Significant Bit (MSB) of the digitized sine and cosine data (MSB S7 and MSB C7 ) of the ADC 44 respectively present on signal paths 44C and 44D are provided to the error correction prom 42B. This, when combined with the 12-bit data from correlators 18A through 18F, provides a 14-bit address to the error correction prom 42B. The error correction prom 42B is programmed with a mathematically generated error correction algorithm, known in the art, which produces an 8-bit error corrected raw coarse frequency data word on signal path 42F. This is the 8 MSB of frequency data prior to temperature correction.
With reference to the digitized sine and cosine data of correlator 18G, these two 6-bit data words are provided to the phase split prom 42E which, based on an analysis of the digitized sine and cosine video data, provides three outputs. More particularly, by using an arc-tangent algorithm, it provides the 5 LSB (Least Significant Bits) of a raw frequency data word indicated as signal path 42G. These 5-bits are attached to the 8-bit raw coarse frequency data word of signal path 42F to form the 13-bit raw frequency data word. In addition, the phase split prom 42E also generates a coherent threshold and data valid, which is an estimate of the utility of the data and which is indicated as signal path 42H.
In parallel, the temperature video, from the temperature sensor attached to the RF delay line, is digitized, forming a 5-bit temperature data word indicated as signal path 42I. This data is combined by the temperature correction prom 42C with the 13-bit raw frequency data word to generate a.+-.correction which is added to the 13-bit raw frequency data word, to produce a 12-bit corrected frequency data word on signal path 42J which is routed to the output data latch the output of which is signal 60 of FIG. 4. The reason for the reduction from 13-bits to 12-bits is that it is desired to correct for temperature at half the frequency resolution so as to avoid dealing with the temperature boundaries at the digital output.
It should be noted that the actual technique for combining the temperature data with the raw frequency data is to form a 13-bit address for a temperature correction prom 42C, using the 8 MSB of frequency data and the 5-bits of temperature data. This prom 42C output is a.+-.sign bit, plus 7-bit correction data, with the LSB of this data at half the output frequency resolution. This 7-bit data word is added to (or subtracted from, depending on the.+-.sign bit) the 13-bit raw frequency data word in a binary full adder circuit. At the output of the adder circuit, the LSB is ignored, to produce the 12-bit frequency data output on signal path 42J.
While the arrangement 34 of FIG. 4 provides for accurate measured RF frequency data that serves an intended purpose, especially for static temperatures, there is a drawback with the arrangement 34 with regard to dynamic temperature changes. More particularly, in the arrangement 34 there is a time lag for sensing the changes in the temperature that the temperature resistor 48 is subjected to relative to the temperature of the delay device 46. This time lag is created by imperfections of the compound 50 coupling the temperature sensitive resistor 48 to the delay device 46. This time lag produces an error associated with the temperature sensitive resistor 48 and the delay device 46, especially at different temperatures. This time lag effect is particularly noticeable when the RF radio receivers of FIG. 4 are employed in high performance aircraft, where the temperature of the environment in which the RF radio receivers need to operate may change more than 50.degree. C. in less than 5 minutes.
It is desired that an arrangement for measuring the frequency of an applied RF signal be provided that is substantially free from temperatures variations that might otherwise degrade the accuracy of the frequency measurement.
Accordingly, it is a primary object of the present invention to provide for an arrangement for measuring the frequency of an applied RF signal whose accuracy is not degraded by changes in temperature. More particularly, it is of prime importance to the present invention to directly measure the temperature of the delay line carrying the RF signal without the use of indirect device such as sensing resistor connected across the delay line of the correlator and use that measurement to directly compensate for any temperature changes that might otherwise degrade the accuracy of the measurement of the frequency of the applied RF signal.
It is a further object of the present invention to provide a correlator and associated circuitry used in frequency measurements that compensate for temperature variations.
Further still, it is an object of the present invention to provide circuitry used in the reference or delay line path of a correlator that compensates for temperatures variations therein.
Still further, it is an object of the present invention to provide for an arrangement that receives an applied RF signal, digitizes the applied signal, and provides a digitized output signal representative of an accurate frequency measurement of the applied RF signal.
It is another object of the present invention to provide for an arrangement that measures the resistance of the delay path of the correlator and uses that resistance measurement to provide for a signal that is used to compensate for temperature variations.
In addition, it is an object of the present invention to provide for a coaxial delay line in which the resistance of the center conductor and/or the outer shield is measured to provide for a signal that compensates for the temperature variations to which a correlator is subjected.